Monoamine oxidase (MAO) will be assayed in the platelets of controls, schizophrenic patients and selected relatives. In contrast to previous studies, different substrates at varying concentrations, to determine the Michaelis constant (Km) and the maximal velocity (Vmax), will be used. The substrates will include norepinephrine, dopamine, serotonin, Dimethyltryptamine and 5-methoxytryptamine. First, platelet MAO of chronic schizophrenia. Second, platelet MAO from acute schizophrenics will be assayed, and the patients will be followed prospectively for three years to determine to better characterize that enzyme in chronic schizophrenia. Second, platelet MAO from acute schizophrenics will be assayed, and the patients will be followed prospectively for three years to determine whether the results obtained have any value in predicting which patients will recover and which patients will develop the chronic disease. Third, platelet MAO from the immediate relatives with low platelet MAO values are at greater risk to develop the disease than similar relatives with normal platelet MAO values. Standard psychiatric evaluations and conventional MAO enzymatic procedures will be used.